add a workaround for travis

Now, there is a permission error in travis.
    "Cannot write into the `/Users/travis/Library/RubyMotion/build' directory, please remove or check permissions and try again."
    https://travis-ci.org/rubymotion/BubbleWrap/builds/9522953

This way is provided by ProMotion gem
https://github.com/clearsightstudio/ProMotion/commit/40e51fc2713735f7db5bca8a6b837a3466afca2c
Merge pull request #279 from Watson1978/travis

add a workaround for travis